I read this trilogy as a single book. A bit of a slog, but generally worth it. The stories follow on. They are stand-alone to some extent, but you need the back story (and I've since found the other boxset on amazon should be read first). There is a minor cliffhanger to link to the next in the series, too.
I'm thinking about the next ones, but really hoping they will be combined in "volume 2" with a subsequent "good deal"!